Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). In some examples, a portion of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, and/or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling. By reducing displacement under low torque request situations, the engine is operated at a higher manifold pressure, reducing engine friction due to pumping, and resulting in reduced fuel consumption.
As such, VDE engines configured with only port fuel injection systems may have problems during transitions between VDE and non-VDE modes of operation. For example, transient fuel control may be a concern when reactivating cylinders. Deactivated cylinders may take multiple combustion events, following reactivation, to establish an intake port fuel puddle and attain stable combustion. Further, without an established intake port fuel puddle during the transition, fuelling errors may occur, and emissions and drivability issues may increase due to degraded combustion stability. In another example, during a transition from non-VDE mode to VDE mode of operation, it may be impracticable to trap a fresh air charge in deactivated cylinders because of the time needed for the intake port fuel puddle to dissipate. Specifically, the trapped air charge may include a portion of fuel drawn in from the puddle which may lead to partial burn and/or misfire when the charge is sparked upon reactivation. Alternatively, if the trapped air charge with fuel is expelled without being combusted, unburned hydrocarbons in the exhaust may elevate catalyst temperature leading to degradation of the catalyst.
The inventors herein have recognized the above issues and identified an approach to at least partly address the above issues. In one example approach, a method is provided for an engine with at least one deactivatable cylinder. The method comprises decreasing an amount of fuel injected by a port injector while increasing an amount of fuel injected by a direct injector prior to deactivating the cylinder. In this way, a fuel puddle at an intake port of the cylinder may be completely dissipated before deactivation allowing for trapping a fresh air charge within the deactivated cylinder.
In another example, a method comprises: before selectively deactivating a cylinder in response to operating conditions, reducing a first proportion of fuel injected by a port injector while correspondingly increasing a second proportion of fuel injected by a direct injector, and when reactivating the cylinder from deactivation, increasing the second proportion of fuel delivered via the direct injector relative to the first proportion of fuel delivered via the port injector.
As an example, a variable displacement engine (VDE) system may include selectively deactivatable cylinders, wherein each cylinder is configured with each of a port injector and a direct injector. In response to deactivation conditions, such as reduced engine load or torque demand, one or more cylinders may be deactivated and the engine may be operated in a VDE mode. For example, the engine may be operated with half the cylinders deactivated and with the remaining active cylinders operating at a higher cylinder load. Prior to deactivation and before transitioning from a non-VDE mode to a VDE mode, cylinders selected to be deactivated may be operated with an increased proportion of fuel delivered from their respective direct injectors. Simultaneously, the cylinders may receive a lower proportion of fuel delivered from their respective port injectors. In one example, the port injectors may be disabled and the cylinders may receive substantially no fuel from the port injectors. By reducing the proportion of fuel delivered by the port injectors or disabling the port injectors, existing fuel puddles at the intake ports of the cylinders to be deactivated may thus be consumed. In response to the complete depletion of the fuel puddles, direct injectors may be disabled, fresh air may be drawn into the cylinders and the intake and exhaust valves may be closed and deactivated. In this way, a fresh air charge may be trapped within a deactivated cylinder.
In response to reactivation conditions, such as increased engine load or torque demand, the deactivated cylinders may be reactivated and the engine may resume a non-VDE mode of operation wherein all the cylinders are operated at a lower average cylinder load. Herein, the reactivated cylinders may be operated with an increased proportion of fuel from their respective direct injectors and a reduced proportion of fuel from their respective port injectors until fuel puddles are established in their respective intake ports. The quantity of each intake port fuel puddle may be estimated and when a steady state quantity of fuel is reached within an intake port fuel puddle, the respective cylinder may then receive a smaller proportion of fuel from its direct injector and a larger proportion of fuel from its port injector.
In this way, by fueling a reactivated cylinder with an initial higher ratio of direct injection relative to port injection, transient fuel control may be improved allowing for more stable combustion. At the same time, an intake port fuel puddle may be established via the initial, smaller proportion of port injection allowing for a smoother transition to a higher proportion of port fuel injection at a later time with reduced transient fueling errors. Further, by reducing the proportion of port injected fuel prior to deactivation, a fresh air charge with reduced traces of unburned fuel may be trapped within a deactivated cylinder. Further still, this fresh air charge may be expelled in a un-combusted state from the reactivated cylinder without a concern for elevated temperature at the exhaust catalyst (e.g., due to unburned hydrocarbons in the exhaust) and catalyst performance may be enhanced, while stoichiometry can be retained overall by correspondingly running a non-deactivated cylinder rich while expelling the fresh charge. Stoichiometry can be achieved more accurately because the fresh air quantity has a reduced uncertainty in terms of un-burned or partially burned fuel from the puddle. Overall, by controlling fuel injection ratios during engine operation transitions, engine performance and emissions may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.